PROJECT SUMMARY/ABSTRACT This proposal is for an investigator-initiated clinical trial R01. Obesity is a major public health problem, and although short-term weight loss is achievable, individuals often regain the majority of weight that was lost. Current approaches have adjusted behavioral prescriptions and lengthened treatment contact in an effort to address this problem, with modest effect. The investigators of this proposal have developed and piloted a brief intervention based on Acceptance and Commitment Therapy (ACT) that focuses on teaching novel content and skills that promote values clarification and commitment; providing the needed motivation to continue with weight control efforts. In a pilot study, participants (N=102) who recently lost ?5% in a behavioral weight loss program were randomly assigned to receive (a) 5-hour ACT workshop, a 5-hour workshop based on Self- Regulation (SR), or a no workshop control condition (Control), with all groups receiving 3 months of follow-up email contact. The ACT group had greater weight loss at 24 months (7.2%) relative to the SR (4.2%; Cohen?s d=.39) and Control (-1.1%; d=.73). These results represent a potential paradigm shift for improving weight loss maintenance towards teaching completely novel skills delivered in brief format, warranting study in a larger trial. Therefore, the proposed R01 project is a randomized controlled clinical trial to compare the efficacy of an ACT intervention and a SR intervention on weight loss maintenance over a 30-month period. All participants will first complete a well-validated online weight loss intervention (months 1-3 of the study). Participants who lose ?4 kilograms of initial weight will then be randomly assigned to receive ACT or SR, with both conditions consisting of three face-to-face, group-based intervention meetings (10 hours total) and weekly email contact for 6 months. The ACT intervention will target a novel theoretically derived intervention target, values- consistent behavior, which will help align weight loss goals with personal values and foster internal motivation to continue with weight control efforts. Assessments will be at baseline, post-weight loss/pre-randomization, and then 6, 12, 18, 24, and 30-month follow-up (months from randomization). We hypothesize that the ACT group will show significantly better weight loss maintenance (i.e. lower mean weight gain) at 30 months as compared to SR. The primary site for this award is the Weight Control and Diabetes Research Center (WCDRC). The WCDRC is part of the Centers for Behavioral and Preventive Medicine at The Miriam Hospital in Providence, Rhode Island, and the Department of Psychiatry and Human Behavior at the Alpert Medical School of Brown University. This study is significant because it addresses weight loss maintenance ? a critical barrier to treating obesity ? and results of the study could be used to improve long-term weight loss outcomes and associated health benefits for treatment seeing overweight and obese adults.